A load sensing system (LUDV) with proportional flow rate reduction for all hydraulic loads, if the volumetric current of hydraulic fluid provided by the pump is insufficient for supplying all the hydraulic loads, is known from the state of the art. This regulation strategy is implemented by pressure compensators, located downstream of the spool valve. The pressure compensators maintain a constant difference in pressure, and thus one independent of the load across the A-B control edge on the load side.
Negative flow control (NFC) is also a very common hydraulic control system, in which spool valve deflection entails a reduction in the volumetric current in the open center duct and thus a reduction in the volumetric control current used at the negative flow control valve. In the negative flow control valve, the change in the volumetric control flow is converted into a difference in pressure, which is used as a signal for controlling pumps. Unlike load sensing systems, no load compensation is carried out by the pressure compensators.
Moreover, the load sensing system (PMSIII) is known in the state of the art from patent specification DE 23 64 282 C3. It is characteristic of this control system that the pumps are set to a greater volumetric displacement as control pressure increases, on the “positive control principle”. The cross-section of control edges C1 and C2 then decreases on the pump side, with the volumetric current of fluid accumulating in front of said control edges C1 and C2. Simultaneously, the control edges A and B on the load side start to open, causing both the pressure of the loads and the system pressure accumulated by control edges C1 and C2 to act upon the load holding valves until the system pressure opens them so that the volumetric current of fluid can flow through the increasing cross sections of the control edges A and B on the load side.
Following the comparison of the control systems for construction vehicles most commonly used in practice, it will be clear that, in accordance with the load-sensing system (PMSIII) mentioned, a load without additional components can be supplied with a volumetric current of fluid by means of the 8/3 way spool valve and the corresponding block structure, produced by the combination of two pumps in the block. By using two pumps, by which hydraulic fluid can be applied in series to the spool valves, the block structure permits two or, in conjunction with the stewing radius, even three system pressures to be used for operation. A further advantage, which may not be underestimated, is that various system matches can be achieved by parallel actuation of the two pumps and spool valve, producing different machine responses, depending upon use of the construction vehicle or the wishes of the customer. The operator retains sensitivity to the digging process because the individual hydraulic functions are not actuated with load pressure compensation.
Hydraulic decoupling of the individual functions is also accomplished simply, by the C control edges of the spool valves, which close the pump ducts as a function of the stroke when actuating or deflecting the spool valve. Neither are any valve-type pressure compensators required, which is firstly energetically efficient and secondly produces a hydraulic control system with a simple structure.
Despite this mature control system in accordance with patent DE 23 64 282 C3, some disadvantages arise from the type of main control block architecture. For example, the series arrangement of spool valves in conjunction with the C edges has the disadvantage that loads may be under-supplied. In particular, there is a danger of restriction of the intended function of loads under-supplied with hydraulic fluid by spool valves located in the middle of the main control block. Such partially restricted supply of loads is further aggravated by the restricted ability to control options or additional functions, particularly if a specific volumetric current is required. In the past, a specific volumetric current had to be set when an additional function, e.g. a cutter or magnetic system, was used, by means of setting one of the two main pumps to a specific volumetric displacement. This greatly restricted the extent of the function of the remaining hydraulic system, as one of the two main pumps was only available exclusively for this additional function.
It is not technically possible to extend the existing main control block to accommodate special functions at the end by means of so-called sandwich elements. Special functions therefore have to be integrated by means of additional valves and hoses, entailing not inconsiderable technical outlay and the associated costs.